Directionally solidified eutectic type alloys with aligned delta phase

ABSTRACT

A nickel alloy containing columbium and/or tantalum and chromium and/or aluminum and displaying eutectic-type behavior is unidirectionally cast under conditions resulting in coupled growth, providing a lamellar biphase or multiphase microstructure displaying about 25-40 percent of a substantially aligned delta phase, Ni3Cb, Ni3Ta or Ni3(Cb,Ta), in a nickel alloy matrix containing chromium and/or aluminum.

United States Patent 1 Lemkey et al.

[ Feb. 19, 1974 DIRECTIONALLY SOLIDIFIED EUTECTIC TYPE ALLOYS WITHALIGNED DELTA PHASE Inventors: Franklin D. Lemkey; Earl R.

Thompson, both of Glastonbury, Conn.

Assignee: United Aircraft Corporation, East Hartford, Conn.

Filed: Jan. 27, 1972 Appl. No.: 221,165

U.S. Cl 75/170, 75/171, 148/32, 148/32.5 Int. Cl. C22c 19/00 Field ofSearch 75/170, 171; 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 3,554,817 l/l97l Thompson148/32 Primary Examiner Richard 0. Dean Attorney, Agent, or Firm-RichardN. James [57] ABSTRACT A nickel alloy containing columbium and/0rtantalum and chromium and/or aluminum and displayingeutectic-typebehavior is unidirectiona lly cast under conditions resulting in coupledgrowth, providing a lamellar biphase or multiphas' microstructuredisplaying about 25-40 percent of a substantially aligned delta phase,Ni Cb, Ni Ta or Ni (Cb,Ta), in a nickel alloy matrix containing chromiumand/or aluminum.

10 Claims, 10 Drawing Figures PATENTED FEB] 9 I974 SHEET 1 BF 8 w w MPAIENTED FEB 1 91974 3.793.010

8HEET3UF8 F'IG.4

PAIENIEB I 3.793.010

' I sum-s 0F 8 I DIRECTIONALLY SOLIDIFIED EUTECTIC TYPE ALLOYS WITH ALIGNED DELTA PHASE BACKGROUND OF THE INVENTION The present inventionrelates to anisotropic castings resultant from the unidirectionalsolidification of nickel-(columbium, tantalum)-(chromium, aluminum)eutectic-type alloys.

It is now known that certain eutectic alloys respond to properunidirectional solidification techniques to produce useful phase-alignedmicrostructres, as described in the patent to Kraft U.S. Pat. No.3,124,452. In the patent to Thompson US. Pat. No. 3,554,817, there isdescribed a particularly promising pseudobinary eutectic alloy occurringbetween the intermetallic compounds Ni,,Al and Ni Cb which responds toplane-front solidification producing a casting characterized by analigned lamellar microstructure. As so cast, this combination providesthe strongest nickelbase alloy known. As is the case with mostnickel-base alloys, however, improvements in the oxidation resistanceare desired, particularly to permit maximum utilization of the strengthcharacteristics of the alloy in the very high temperature ranges. Andeven though in its usual application a given alloy may be provided withsurface protection for increased oxidation-erosion resistance,improvements in corrosion resistance in the underlying substrate arenevertheless desirable.

In a prior patent of the same inventors, Thompson and Lemkey us. Pat;No. 3,564,940, there is described a class of compositions which solidifyaccording to the monovariant eutectic reaction, providing alignedpolyphase structures, including such systems as the ternary alloysidentified as cobalt-chromiumcarbon and nickel-aluminum-chromium., Theadvantage of compositions of this nature is that the desiredmicrostructure can be achieved over a range of compositions within agiven system. This provides a substantial increase in the freedom ofselection of composition permitting increased optimization ofproperties. In a copending application of the same inventors filed Dec.10, 1969, application Ser. No. 883,713, now US. Pat. No. 3 ,67 1,223,the concept has been further developed to include those systemssolidifying according to the multivariant eutectic reaction where two ormore solid phases (n) crystallize simultaneously from a liquidconsisting of (n+2) or more components.

SUMMARY OF THE INVENTION The present invention relates to eutectic-typealloys within a basic nickel-(columbium, tantalum (chromium, aluminum)system which respond to plane-front solidification to providephase-aligned-microstructures comprising a lamellar delta phase in anickel alloy matrix. As directionally solidified, these alloys arecharacterized by a nickel alloy matrix containing up to about 35 weightpercent chromium and/or up to about 4 weight percent aluminum insolid'solution and about 25-40 volume percent of a lamellar Ni Cb, Ni Taor Ni (Cb,Ta) delta phase in alignment therein.

In a preferred composition the alloy, as directionally cast, isnominally characterized by a nickel-chromiumaluminum matrix phasecontaining up to aboutl35 weight percent chromium and up to about 4weight percent aluminum in solid solution with about 25-40 volumepercent of the lamellar delta (8) phase in alignment therein. In aparticular preferred embodiment, the as directionally cast alloy is agehardenable through the precipitation of y (a phase based on theintermetallic compound Ni Al) as a dispersed phase in the y matrix.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of alongitudinal section of a directionally solidified alloy of the presentinvention at the monovariant eutectic composition comprising, by weight,73 percent nickel, 21 percent columbium, and 6 percent chromium (200 Xbefore reduction).

FIG. 2 is a similar photomicrograph of another directionally solidifiedalloy at the monovariant eutectic composition comprising, by weight, '71percent nickel, 20 percent columbium, and 9 percent chromium.

FIG. 3 is a photomicrograph of a longitudinal section of a directionallysolidified alloy of the present invention at the monovariant eutecticcomposition comprising, by weight, 76.4 percent nickel, 20.8 percentcolumbium, and 2.8 percent aluminum.

FIG. 4 is a photomicrograph of a transverse section of a bivarianteutectic composition from the NiCb-- CrAl system wherein 7' phaseprecipitation has occurred.

FIG. 5 is a photomicrograph of a longitudinal section of an alloy at thecomposition comprising, by weight, 60 percent nickel, 30 percenttantalum and 10 percent chromium, as directionally solidified.

FIG. 6 is a graph comprising the liquidus properties in thenickel-columbium-chromium system.

FIG. 7 is a graph showing the eutectic troughs and single phase fieldboundaries in the nickel-rich corner of the nickel-columbium-aluminumdiagram.

FIG. 8 is a representation showing a polythermal projection of thenickel-columbium-chromium-aluminum diagram which illustrates thebivariant surface wherein the three-phase equilibriumof the type L 'y 8occurs.

FIG. 9 is a graph demonstrating the response of a y 8 monovarianteutectic alloy and a 'y 5 bivariant eutectic alloy to cyclic furnaceoxidation at 2,000F, as compared to certain other alloy systems.

FIG. 10 is a graph comparing the sulfidation erosion resistance of abivariant nickel-columbium-chromiumaluminum alloy withother hightemperature alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 7 As previously described, thearticles to which the present invention has reference are thedirectionally solidified alloy castings of monvariant or multivarianteutectic composition which display about 25-40 volume percent of analigned lamellar 6 phase embedded in a 'y matrix. The 8 phase includesnot only the Ni Cb and Ni Ta intermet'allic but also, where both colum-I a disc-shaped phase which is believed to be the 7" phase, a tetragonalcompound of nickel and columbium.

The 8 phase alignment in these alloys is clearly shown in FIGS. l-S.FIGS. 1-3 and 5 show various articles of monovariant ternary eutecticcomposition as directionally solidified. FIG. 4 is a quarternarynickel-columbium-chromium-aluminum alloy, a preferred embodimentexhibiting bivariant eutectic behavior in solidification. Particularnote will be taken in FIG. 4 of the strengthening cuboidal 'y'precipitate in the matrix phase.

The articles thus described are those achieved by unidirectionalsolidification of alloys in the nickel- (columbium, tantalum)-(chromium,aluminum) compositional system exhibiting monovariant or multivariantbehavior upon solidification with an aligned 8 phase. In thiscompositional system, the columbium and tantalum appear, generallyspeaking, to be broadly mutually substitutional or essentially an atomicbasis. Either chromium or aluminum or both may be present in the alloy,providing up to about 35 weight percent chromium and up to about 4weight percent aluminum in the matrix phase as cast.

The general compositional limitations in terms of the melt can best beascertained by reference to FIGS. 6-8. FIG. 6 details the liquidusproperties in the nickel-columbium-chromium ternary system while FIG. 7illustrates the eutectic troughs and single phase field boundaries inthe nickel-rich corner of the nickelcolumbium-aluminum diagram.

The compositions in the quartemary nickel-columbium-chromium-aluminumalloy system of this invention solidify according to the bivarianteutectic reaction, which describes the compositions, defined by aliquidus surface, wherein two solidphases separate from the liquid uponsolidification.

Referring to FIG. 8, this surface (shaded in the drawing) will be seento be bounded by eutectic troughs extending, in thenickel-columbium-chromium system between but exclusive of the binaryeutectic NiNi Cb and the ternary eutectic NiCbCr Ni Cb, and, in thenickel-columbium-aluminum system, between but exclusive of the binaryeutectic NiNi Cb and the ternary eutectic Ni-Ni AlNi Cb.

Compositions on the monovariant eutectic troughs have been found toprovide the desired phase alignment and characteristic properties. Thealigned longitudinal microstructures of the compositions compris ing, byweight, 73 percent nickel, 21 percent columbium (297C), and 6 percentchromium, and 71 percent nickel, 20 percent columbium (109C), and 9percent chromium are shown in FIGS. 1 and 2, respectively. The alignedlongitudinal microstructure of a composition comprising, by weight, 76.4percent nickel, 20.8 percent columbium, and 2.8 percent aluminum, isshown in FIG. 3. That of the alloy comprising, by weight, 60 percentnickel, 30 percent tantalum and 10 percent chromium in longitudinalsection is seen in FIG. 5. The 8 phase is Ni Ta and the y phaseis asolid solution of nickel and chromium.

The transverse microstructure of the quaternary alloy comprising byweight, 68 percent nickel, 20.6 percent columbium, 9.2 percent chromiumandv 2.2 percent aluminum, as shown in FIG. 4, provides clear evidenceof the maintenance of the physical aspects of the desired lamellarmicrostructure. Within the y matrix there are seen precipitates of botha cuboidal 7, an intermetallic compound based on Ni Al, and a discshapedphase which is believed to be 7" frequently observed in the superalloyscontaining significant amounts of both aluminum and columbium. As willbe noted, the lamellar Ni Cb(8) phase comprises approximately 30 volumepercent of the microstructure.

Unidirectionally solidified articles of the Alloy 297C composition, whentested in the direction of phase alignment at both room temperature andat 2,000F, have demonstrated significant strengthening attributable tothe aligned microduplex structure. This alloy has exhibited a tensilestrength of 171,000 psi at room temperature. The strengths of this alloysolidified at 2 cm/hr and 10 cm/hr were at 2,000F, respectively, 60,200psi with a failure strain of 6 percent, and 74,400 psi with a failurestrain of 8.5 percent. These values of tensile strength approach thosereported for the nickelcolumbium-aluminum eutectic system of U.S. Pat.No. 3,554,817, supra, and represent a significant strength increase overthe conventional nickel-base superalloy systems. In addition, ashereinafter described in greater detail, additional increases instrength and ductility are attainable by suitable alloy modificationwithin the basic bivariant or multivariant alloy solidificationmechanism.

In the nickel-chromium type alloys, the corrosion resistance inoxidizing atmospheres is strongly or primarily dependent upon thechromium content of the system. Since oxidation of these alloys, asrepresented by the scaling rate, exhibits a minimum in corrosionsusceptibility as the chromium content is varied, biphase alloys in thenickel-chromium-columbium system of a composition defined by themonovariant eutectic trough may be selected to provide the maximumoxidation resistance for a given environment. This flexibility incomposition adjustment is inherent in the present invention.

Similarly, aluminum is recognized to act beneficially in increasingoxidation resistance of castings. Thus, chromium and aluminum separatelyas in the nickel-columbium-chromium and nickel-columbiumaluminum alloys,respectively, or in combination as in thenickel-columbiumchromium-aluminum alloys may significantly improve theoxidation properties.

The oxidation properties of eutectic alloys at compositions comprising,by weight, 69 percent nickel, 19 percent columbium, 12 percent chromiumhaving a matrix composition (7) of, by weight, 66 percent nickel, 27percent chromium, and 7 percent columbium, and 69.9 percent nickel, 20.4percent columbium, 9.2 percent chromium, 0.5 percent aluminum wereascertained at 2,000F. The results are graphically illustrated in FIG.9. These alloys are demonstrably superior in terms of their oxidationresistance to both the NiNi Cb and Ni Al Ni Cb eutectic alloys, andcompare favorably with the conventional nickel-base superalloys of highcorrosion resistance.

- The results of a burner ring thermal cycle test on an alloy test barat the composition comprising, by weight, 69.7 percent nickel, 20.2percent columbium, 9.1 percent chromium, and 1.0 percent aluminumtogether with various other nickel-base super alloys is shown in FIG.10. The specimens in this test were held in cycles of 1,750F for threeminutes followed by heating to 2,050F for two minutes and cooling to600F while rotating in a jet burner using JP-SR fuel with 3.5 ppmsnythetic sea salt operating at Mach 0.3. Besides the excellent relativeranking of the alloy from this test, of as great an interest was thefact that the material suffered no decrease in tensile properties after219.5 hours of thermal cycling. 'After vane cyclic sulfidation testingthe erosion bar was machined into a tensile specimen such that the areaof greatest erosion was within the gage length and tested at 2,000F inair at a strain rate of 0.02 per minute. The ultimate tensile strengthand strain to failure were 53,400 psi and 14.2 percent respectivelywhile the strength and failure strain were 52,500 psi and 17.9 percentprior to rig testing.

Alloys in the system of the present invention are tolerant of theselected addition of other materials to the basic ternary andquarternary compositions. In general, deviations from or additions tothe composition loci defining the monovariant and bivariant eutecticreaction may be made providing, however, that in kind and quantity theydo not interfere with the'basic coupled growth mechanism by which thelamellar biphase microstructure is produced. In this respect, theeutectictype alloys, in general, have been found to be tolerant of theaddition of sometimes rather substantial quantities of other elements,usually selected to provide or promote the development of a particularproperty or characteristic in the casting.

A number of ingredients are known to have a profound effect on alloyproperties, even though present only in very small amounts. Yttrium andthe rare earth elements, for example, in quantities as low as 0.03weight percent, have been found to promote oxide adherence to thenickel-base superalloys. In some cases, quantities of boron, carbon,hafnium and zirconium have been found to promote creep rupture ductilityand, in some cases, cause a pronounced reduction of the tendency of suchmaterials to grain boundary oxidation. As previously mentioned, however,those modifications to the basic ternary or quarternary alloys whichprovide advantageous results and do not interfere with the developmentof the desired lamellar microstructure may be included therein.Generally, the solidification rates, typically about 05-10 cm/hr,usually associated with the unidirectional solidification of theeutectic-type alloys are also applicable to the present invention. Thegreater degree of dispersed phase alignment normally occurs in the caseof solidification from the ideally planar liquid-solid interface.However, a substantial and, obviously in many cases, satisfactory phasealignment is. also achieved from the cusped interface which may occur atthe'higher solidification rates.

Accordingly, the invention in its broader aspects is not limited to thespecific methods, compositions and examples herein described, butnumerous modifications, alterations and additions may be madetheretowithout departing from the true spirit of the invention and withoutsacrificing its chief advantages.

We claim:

1. A directionally solidified alloy casting having an overallcomposition selected from the group consisting of the monovariant andmultivariant eutectic nickelbase alloys solidifying in a polyphasestructure consisting of, a 8 phase of the Ni M type where M representsat least one element selected from the group consisting of columbium andtantalum and a 'y phase consisting of a nickel-base alloy containing atleast one element selected from the group consisting of chromium andaluminum, and characterized by an anisotropic microstructure having the8 phase solidified as lamellae in substantial alignment in a matrixconsisting essentially of the 7 phase.

2. A casting according to claim 1 wherein:

the 8 phase comprises about 25-40 volume percent of the casting.

3. A casting according to claim 1 wherein:

the y phase is a nickel-base alloy containing at least one elementselected from the group consisting of chromium and aluminum in an amountof, as cast, not exceeding, by weight, about 35 percent chromium andabout 4 percent aluminum.

4. A casting according to claim 3 wherein:

the 'y phase contains both chromium and aluminum and the y phase, asaged, contains a fine y strengthening phase precipitate.

5. A directionally solidified alloy casting of substantially monovarianteutectic composition characterized by an anisotropic microstructurehaving a lamellar Ni Cb phase substantially aligned in a nickel-basealloy matrix containing in solid solution an element selected from thegroup consisting of chromium and aluminum.

6. A casting as in claim 5 wherein:

the Ni Cb phase occupies about-25-40 volume percent of the casting.

7. A directionally solidified alloy casting of substantially monovarianteutectic composition characterized by an anisotropic microstructurehaving a lamellar Ni Ta phase substantially aligned in a nickel-basealloy matrix containing in solid solution an element selected from groupconsisting of chromium and aluminum.

8. A casting as in claim 7 wherein:

the Ni Ta phase occupies about 2540 volume percent of the casting.

9. A directionally solidified alloy casting of substantiallymultivariant eutectic composition characterized by an anisotropicmicrostructure having about 2540 volume percent of a lamellar Ni Mphase, where M is selected from the group consisting of tantalum andcolumbium, embedded in substantial alignment in a nickel-base alloymatrix containing in solid solution as cast up to 35 weight percentchromium and up to about 4 weight percent aluminum. I

10. A casting according to claim 9 wherein:

the matrix phase also contains fine particles of a strengthening 'yprecipitate.

2. A casting according to claim 1 wherein: the delta phase comprisesabout 25-40 volume percent of the casting.
 3. A casting according toclaim 1 wherein: the gamma phase is a nickel-base alloy containing atleast one element selected from the group consisting of chromium andaluminum in an amount of, as cast, not exceeding, by weight, about 35percent chromium and about 4 percent aluminum.
 4. A casting according toclaim 3 wherein: the gamma phase contains both chromium and aluminum andthe gamma phase, as aged, contains a fine gamma '' strengthening phaseprecipitate.
 5. A directionally solidified alloy casting ofsubstantially monovariant eutectic composition characterized by ananisotropic microstructure having a lamellar Ni3Cb phase substantiallyaligned in a nickel-base alloy matrix containing in solid solution anelement selected from the group consisting of chromium and aluminum. 6.A casting as in claim 5 wherein: the Ni3Cb phase occupies about 25-40volume percent of the casting.
 7. A directionally solidified alloycasting of substantially monovariant eutectic composition characterizedby an anisotropic microstructure having a lamellar NI3Ta phasesubstantially aligned in a nickel-base alloy matrix containing in solidsolution an element selected from the group consisting of chromium andaluminum.
 8. A casting as in claim 7 wherein: the Ni3Ta phase occupiesabout 25-40 volume percent of the casting.
 9. A directionally solidifiedalloy casting of substantially multivariant eutectic compositioncharacterized by an anisotropic microstructure having about 25-40 volumepercent of a lamellar Ni3M phase, where M is selected from the groupconsisting of tantalum and columbium, embedded in substantial alignmentin a nickel-base alloy matrix containing in solid solution as cast up to35 weight percent chromium and up to about 4 weight percent aluminum.10. A casting according to claim 9 wherein: the matrix phase alsocontains fine particles of a strengthening gamma '' precipitate.